ALLOWANCE FOR CREDIT LOSSES - Schedule of Allowance for Credit Losses for HTM Debt Securities (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Debt Securities, Held-to-maturity, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses on HTM debt securities at beginning of quarter	$ 87	$ 86
Gross credit losses	0	0
Gross recoveries	0	3
Net credit losses (NCLs)	0	3
Net reserve builds (releases)	(2)	(8)
Net specific reserve builds (releases)	0	0
Total provision for credit losses on HTM debt securities	(2)	(11)
Allowance for credit losses on HTM debt securities at end of quarter	85	78
Mortgage-backed securities
Debt Securities, Held-to-maturity, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses on HTM debt securities at beginning of quarter	6	3
Gross credit losses	0	0
Gross recoveries	0	3
Net credit losses (NCLs)	0	3
Net reserve builds (releases)	(2)	1
Net specific reserve builds (releases)	0	0
Total provision for credit losses on HTM debt securities	(2)	(2)
Allowance for credit losses on HTM debt securities at end of quarter	4	4
State and municipal
Debt Securities, Held-to-maturity, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses on HTM debt securities at beginning of quarter	75	74
Gross credit losses	0	0
Gross recoveries	0	0
Net credit losses (NCLs)	0	0
Net reserve builds (releases)	4	(5)
Net specific reserve builds (releases)	0	0
Total provision for credit losses on HTM debt securities	4	(5)
Allowance for credit losses on HTM debt securities at end of quarter	79	69
Foreign government
Debt Securities, Held-to-maturity, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses on HTM debt securities at beginning of quarter	4	6
Gross credit losses	0	0
Gross recoveries	0	0
Net credit losses (NCLs)	0	0
Net reserve builds (releases)	(2)	(1)
Net specific reserve builds (releases)	0	0
Total provision for credit losses on HTM debt securities	(2)	(1)
Allowance for credit losses on HTM debt securities at end of quarter	2	5
Asset-backed
Debt Securities, Held-to-maturity, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses on HTM debt securities at beginning of quarter	2	3
Gross credit losses	0	0
Gross recoveries	0	0
Net credit losses (NCLs)	0	0
Net reserve builds (releases)	(2)	(3)
Net specific reserve builds (releases)	0	0
Total provision for credit losses on HTM debt securities	(2)	(3)
Allowance for credit losses on HTM debt securities at end of quarter	$ 0	$ 0